Touch panels such as for example digitizers and analog resistive touch screens that make use of one or more tensioned membranes, are known in the art. Tensioned touch panels of this nature typically include a conductive membrane that is stretched tautly over and spaced from a conductive substrate. When a pointer is used to contact the tensioned membrane with sufficient activation force, the tensioned membrane deflects and contacts the conductive substrate thereby to make an electrical contact. Determining voltage changes induced by the electrical contact allows the position of pointer contact on the tensioned touch panel to be determined.
In order for such tensioned touch panels to work effectively, the spacing between the tensioned membrane and the conductive substrate must be maintained so that the tensioned membrane only contacts the conductive substrate when a pointer contact is made on the tensioned membrane.
As will be appreciated, over time the tensioned membrane may sag creating slack in the tensioned membrane. Changes in environmental conditions such as humidity and/or temperature may also cause the tensioned membrane to expand resulting in slack developing in the tensioned membrane. If the tensioned membrane sags or expands, the slack developed in the tensioned membrane may result in undesirable contact between the tensioned membrane and the conductive substrate. This problem becomes more severe as the size of the touch panel becomes greater.
A number of techniques have been considered to avoid undesirable contact between the tensioned membrane and the conductive substrate. For example, electrically insulating spacer dots may be disposed between the tensioned membrane and the conductive substrate at spaced locations over the active contact area of the touch panel to maintain the spacing between the tensioned membrane and the conductive substrate. U.S. Pat. No. 5,220,136 to Kent discloses a contact touchscreen including such insulating spacer dots.
Although the use of insulating spacer dots maintains separation between the tensioned membrane and the conductive substrate, the use of insulating spacer dots is problematic. In order to maintain separation between the tensioned membrane and the conductive substrate over the active contact area of the touch panel, the insulating spacer dots must be positioned at locations within the active contact area. Thus, the insulating spacer dots interrupt the active contact area of the touch panel. As a result, contacts with the tensioned membrane over insulating spacer dots will not register as contacts since the tensioned membrane cannot be brought into electrical contact with the conductive substrate at those contact points. Also, the use of insulating spacer dots to separate the tensioned membrane and the conductive substrate is expensive. It is also difficult to maintain an even spacing between the tensioned membrane and the conductive substrate over the active contact area using insulating spacer dots.
U.S. Pat. No. 5,838,309 to Robsky et al. discloses a self-tensioning membrane touch screen that avoids the need for insulating spacer dots. The touch screen includes a support structure having a base and a substrate support on which a conductive surface is disposed. A peripheral insulating rail surrounds the conductive surface. A peripheral flexible wall extends upwardly from the base. A conductive membrane is stretched over the conductive surface and is attached to the peripheral flexible wall. The insulating rail acts to space the conductive membrane from the conductive surface. To inhibit sagging and maintain tension on the conductive membrane, during assembly of the touch screen the conductive membrane is attached to the flexible wall when the flexible wall is in a pretensioned state. In the assembled condition, the flexible wall is biased outwardly and downwardly. As a result, tension is continuously applied to the conductive membrane by the flexible wall thereby to inhibit sagging of the conductive membrane.
U.S. Pat. No. 6,664,950 to Blanchard discloses a resistive touch panel having a removable, tensioned top layer and a base plate. The touch panel may be situated relative to a display screen such that an air gap exists between the base plate and the display screen. The top plate includes a transparent, flexible substrate having a hard transparent coating, one or more anti-reflective coatings and an anti-fingerprint coating thereon. The underside of the substrate is spaced from the upper surface of the base plate by an air gap. To prevent wrinkling of the top plate, a stiff frame is bonded to the anti-fingerprint coating. The stiff frame maintains tension in the top plate despite temperature changes.
Although the above references show touch panels having mechanisms to maintain tension in the conductive membrane, manufacturing and labour costs are associated with these tensioning mechanisms. Accordingly, improvements in tensioned touch panels to maintain the spacing between the tensioned membrane and the conductive substrate are desired.
It is therefore an object of the present invention to provide a novel tensioned touch panel and method of making the same.